Eyes Full of Teeth
Updated: May 17, 2021
In the past, scientists noted that the eyes of the whale shark (Rhinocodon typus) are positioned in either side of the head and project significantly from the "orbit" (aka eye-socket). They seem to really stick out! So, we wondered how the whale shark did not commonly suffer mechanical damage to its eyes when swimming amongst debris in the water. Well, scientists have recently made a truly remarkable discovery... whale sharks have teeth in their eyes! I know, this sounds like science fiction (or completely horrifying)... allow me to explain...
Whilst sharks look smooth from a distance, up close their skin is actually very rough. All sharks, including whale sharks, have modified teeth all over their bodies, called "dermal denticles", which form a skin of protective armour. Dermal denticles are basically tiny, hard scales, which lay as interlocking plates, oriented from nose to tail, to reduce drag when swimming through the water. They give the skin strength, whilst retaining flexibility and so are important for protecting the shark from parasites and from sharp rocks or coral (Tomita et al, 2020). To learn more, you can check out Sand Paper Shark Skin.
Dermal denticles are found in many different shapes and sizes in different areas of the body, and vary greatly between different species. However, no other shark species is known to have dermal denticles in their eyes. Many other species of sharks have "nictating membranes" which, like an eyelid, can be drawn over the eye for protection. Where this is absent, some sharks, like the great white (Carcharodon carcharias), are able to roll the eye backwards, in order to shelter the delicate parts and expose the toughened back of the eyeball.
Whale sharks have previously been witnessed rolling their eyes, but a recent study has found that this is more pronounced than we previously thought. Not only can whale sharks roll their eye, but they can retract the eyeball back into the orbital socket to protect it from damage (Tomita et al, 2020).
But, whilst studying this mechanism, the scientists also discovered that whale sharks have dermal denticles in their eyes which also provide them protection from damage. These denticles contrast to those found across the skin of the shark, as they have an 'oakleaf-like' shape and differ in their surface texture, but like dermal denticles are, in fact, modified teeth. Each eye denticle has a hardened surface, like tooth enamel, making it strong and as many as 3,000 individual dentcles are arranged around the eye. This means that the whole surface of the eye surrounding the iris is armoured, which protects the delicate ocular structures from damage if something strikes them (Tomita et al, 2020).
This is a fascinating finding, because no other sharks are known to have eye denticles, even those which are very closely related to the whale shark. However, several species of extinct sharks have been found to have similar eye armour. This suggests that eye denticles are unique to whale sharks today, but may have been prevalent amongst other sharks species millions of years ago (Tomita et al, 2020).